This invention relates to optical fiber submarine cable and a method of making an optical fiber submarine cable.
As used herein, "submarine cable" means underwater cable even if it is not used in a submarine detection systems.
When detecting submarines by use of sonobuoys, it is desirable to lay long lengths of fiber optic data transmission cable in the deep ocean. The use of special cable laying ships to lay the cable is often impractical in that an adversary or potential adversary may easily detect the laying of the cable. Other vehicles such as aircraft, ordinary ships (i.e., ships which are not easily discerned as including cable laying equipment), or submarines are highly preferred in laying the cable between the submarine-detecting sonobuoys. Long lengths of fiber optic data transmission cable including single optical fiber cables may be laid into the deep ocean for other purposes.
The long cable lengths and the avoidance of detection of the cable both require that the cable be of a very small diameter. Light weight and high information bandwidth are also required. In addition, the cable must have certain characteristics to render it suitable for use on the ocean bottom. In order to span chasms reliably, the fiber optic cable must have a high longitudinal stiffness (tensile modulus) as well as a high sink rate. The high or fast sink rate prevents ocean currents from disrupting the cable position during deployment of the cable.
An efficient submarine detection system may require thousands of kilometers of cable. Therefore, low attenuation, inexpensive, simply designed cable is desirable. The low attenuation characteristic is quite desirable in that it allows for wide spacing between different repeater stations.
Some previous optical fiber submarine cables have used a hermetic tube to surround and enclose the optical fibers. The tube prevents moisture from weakening the fiber. The hermetic tube additionally prevents pressure from causing microbending of the fiber which will in turn attenuate or weaken the optical signal. In some submarine cables, the tube is made of highly conductive copper or aluminum and serves to supply electrical power to optical repeaters spaced along the cable. When the tube is made of conductive material, it is enclosed within electrical insulation to insulate it from the water.
The use of a tube with the cable is quite costly. Additionally, the tube may take on a permanent elongation following the application of tension. This permanent elongation may cause residual stress to be applied to the optical fibers inside of the tube, thereby greatly decreasing their life expectancy. Moreover, the tube may be prone to cracking or necking down (decrease in diameter due to axial stretching). Either of these may cause strain on the optical fibers within the tube. The use of a continuous metal tube is also limited by high equipment costs as well as the low yield strength limitation inherent in the tube forming process. Strengthening requires expensive multi-step processing.
The cable need not be hermetically sealed by a tube. If the cable is kept at low strain, not in excess of 25% of proof test, the presence of water outside the fiber optic buffer will not reduce lifetime of the cable below acceptable levels.
Various previous design approaches have heretofore been used, but have been deficient in their ability to simultaneously control tensile modulus, mass, diameter, attentuation, and cost.
The use of a "pultruded" fiberglass protection for optical fibers entails the disadvantages of high cost, low tensile modulus of glass, and the tendency towards longitudinal cracking.
The use of polyethylene extrusion with embedded wires is limited by high equipment cost and low content of steel wire resulting in low tensile modulus.
The following U.S. patents show various designs which have heretofore been used for cables:
______________________________________ U.S. Pat. No. Inventor Issue Date ______________________________________ 4,072,398 Larsen et al. Feb. 7, 1978 4,097,119 Kumamaru et al. Jun. 27, 1978 4,110,001 Olszewski et al. Aug. 29, 1978 4,146,302 Jachimowicz Mar. 27, 1979 4,154,783 Jackson May 15, 1979 4,155,963 de Vecchis et al. May 22, 1979 4,156,104 Modello May 22, 1979 4,160,872 Lundberg et al. Jul. 10, 1979 4,199,224 Oestreich Apr. 22, 1980 4,239,336 Parfree et al. Dec. 16, 1980 4,278,835 Jackson Jul. 14, 1981 4,317,000 Ferer Feb. 23, 1982 4,324,575 Levy Apr. 13, 1982 4,341,440 Trezeguet et al. Jul. 27, 1982 4,359,598 Dey et al. Nov. 16, 1982 4,371,234 Parfree et al. Feb. 1, 1983 ______________________________________
The Larsen et al. patent discloses a communication cable using optical fibers and an outer coating. The optical fibers are loosely twisted together and provided with tension-relieving wires and sheathed to form a cable with interspaces filled with lubricants.
The Kumamaru et al. patent shows an optical fiber cable using cushioning layers made of nonwoven plastic fabric or foamed plastic. An outer sheathed layer made of a composite of metal and plastic is also used.
The Olszewski et al. patent discloses an optical fiber cable construction including a seam welded metalic tube to protect the optical fiber. The optical fiber is placed within a helical channel in a core element.
The Jachimowicz patent discloses an optical fiber cable having optical fibers helically wound within a welded metal tube. When the cable is stretched, the fibers remain relaxed due to their helical construction.
The Jackson U.S. Pat. No. '783 shows a cable with optical fibers inside two concentric tubes and having steel wires wrapped around the outer tube (aluminum water barrier) in two oppositely wound layers.
The de Vecchis et al. patent shows different methods of making optical fibers and including various extrusion steps.
The Mondello patent discloses a submarine cable including optical fibers and cable strength members including a central filament and layers of stranded steel wires separated from the central filament by an insulating core member. A metallic tube surrounds the layers of stranded steel wire and provides a DC path for powering optical repeaters and a hermetic moisture-barrier for the fiber.
The Lundberg et al. patent discloses a floating cable system including a metallic protective layer around inner electrical conductors.
The Oestreich patent discloses an optical fiber cable including helical extending chambers in which the optical fibers are disposed. Additionally, an apparatus for constructing the cable is disclosed.
The Parfree et al. U.S. Pat. No. '336 shows an optical fiber communication cable including optical fibers within a copper tube. A plastic layer surrounds the copper tube and strength members are disposed around the plastic layer. An outer sheath surrounds the strength members.
The Jackson U.S. Pat. No. '835 shows an optical fiber submarine cable wherein the optical fibers are disposed in petroleum jelly within a copper or aluminum tube. Polycarbonate surrounds the tube and in turn is surrounded by an aluminum water barrier. Inner and outer layers of steel wires are wound in opposite directions around the aluminum water barrier. Mylar tape and polyethylene layers surround the outer wire layers. The inner and outer wires press against each other and against the aluminum water barrier to make the cable rigid when it is under hydrostatic pressure.
The Ferer patent discloses a cable including a plurality of electrically conductive wires or a bundle of optical fibers surrounded by an extrusion of plastic. An inner layer of KEVLAR fiber strands surrounds the plastic and is oppositely wound from an outer helical layer of KEVLAR and nylon fiber strands, separated from the inner layer by a thin plastic film. A braided outer jacket or covering surrounds the outer layer.
The Levy patent discloses the use of ultraviolet cured polymer to coat an optical fiber.
The Trezeguet et al. patent discloses a submarine optical fiber cable including a drawn copper tube having a longitudinal weld and surrounding the optical fibers. Liquid is disposed within the tube, whereas its outside is surrounded by a polyolefin outer sheath, for example polyethylene.
The Dey et al. patent shows an overhead electric transmission system including an optical fiber or optical fiber bundle. Different layers of wires surround a core in which the optical fibers are disposed. Adjacent layers of wires are wound in opposite directions.
The Parfree et al. U.S. Pat. No. '234 discloses a submarine optical fiber cable including optical fiber within an aluminum tube surrounded by an inner layer of armouring wires and a polyetheylene layer. This in turn is surrounded by an additional layer of low density polyethylene having a bedding material such as jute or polypropylene fibers around it. Just outside the jute or polypropylene fiber layer are outer steel armor wires followed by an additional layer of jute or polypropylene fibers.
The following documents disclose various other cable designs:
______________________________________ Document Publication Date ______________________________________ UK Patent Appln. April 21, 1982 2,085,187A UK Patent Appln. March 12, 1980 2,029,048A "Submarine Optical March 1, 1984 Fiber Cable: Development and Laying Results" by Kojima et al - in APPLIED OPTICS, Vol. 21, No. 5, pgs. 815-821. ______________________________________
The UK published patent application 2,029,048A discloses an optical fiber submarine cable including a power supplying and pressure resisting layer which may be formed from a tape-like material into a pipe-like configuration. Tension resisting wires are wound around the power supplying pressure resisting layer and in turn are surrounded by insulation. An outer sheath surrounds the insulation. The wire layer is used as a surge attenuator since it is made of lower conductivity material than the power supplying layer.
UK patent application 2,085,187A discloses an optical fiber cable construction including one or more optical fibers disposed within a conductive tube and surrounded by reinforcing elements. The reinforcing helically wound elements are in turn surrounded by conductive elements wound in the opposite direction.
The magazine article "Submarine Optical Fiber Cable: Development and Laying Results" discloses various design criteria for submarine optical fiber cable. Additionally, it shows a torqueless armoring structure wherein a cable core is surrounded by inner and outer armoring layers of oppositely wound wires.